feat(config): support object-style fallback_models with per-model settings

Add support for object-style entries in fallback_models arrays, enabling
per-model configuration of variant, reasoningEffort, temperature, top_p,
maxTokens, and thinking settings.

- Zod schema for FallbackModelObject with full validation
- normalizeFallbackModels() and flattenToFallbackModelStrings() utilities
- Provider-agnostic model resolution pipeline with fallback chain
- Session prompt params state management
- Fallback chain construction with prefix-match lookup
- Integration across delegate-task, background-agent, and plugin layers

src/plugin/event.test.ts

@@ -4,6 +4,7 @@ import { createEventHandler } from "./event"
 import { createChatMessageHandler } from "./chat-message"
 import { _resetForTesting, setMainSession } from "../features/claude-code-session-state"
 import { clearPendingModelFallback, createModelFallbackHook } from "../hooks/model-fallback/hook"
+import { getSessionPromptParams, setSessionPromptParams } from "../shared/session-prompt-params-state"
 
 type EventInput = { event: { type: string; properties?: unknown } }
 
@@ -441,6 +442,45 @@ describe("createEventHandler - event forwarding", () => {
 		expect(disconnectedSessions).toEqual([sessionID])
 		expect(deletedSessions).toEqual([sessionID])
 	})
+
+	it("clears stored prompt params on session.deleted", async () => {
+		//#given
+		const eventHandler = createEventHandler({
+			ctx: {} as never,
+			pluginConfig: {} as never,
+			firstMessageVariantGate: {
+				markSessionCreated: () => {},
+				clear: () => {},
+			},
+			managers: {
+				skillMcpManager: {
+					disconnectSession: async () => {},
+				},
+				tmuxSessionManager: {
+					onSessionCreated: async () => {},
+					onSessionDeleted: async () => {},
+				},
+			} as never,
+			hooks: {} as never,
+		})
+		const sessionID = "ses_prompt_params_deleted"
+		setSessionPromptParams(sessionID, {
+			temperature: 0.4,
+			topP: 0.7,
+			options: { reasoningEffort: "high" },
+		})
+
+		//#when
+		await eventHandler({
+			event: {
+				type: "session.deleted",
+				properties: { info: { id: sessionID } },
+			},
+		})
+
+		//#then
+		expect(getSessionPromptParams(sessionID)).toBeUndefined()
+	})
 })
 
 describe("createEventHandler - retry dedupe lifecycle", () => {